1. Field of the Invention
This invention relates generally to canceling echoes seen by transceivers when communicating data and/or voice signals, and, more particularly, to a current sensing echo cancellation device.
2. Description of the Related Art
Echo cancellation techniques are commonly employed in telephony applications to provide enhanced signal quality. Echo cancellation has been used in both voice and data applications. In general terms, a portion of the signal transmitted on a telephone line, otherwise known as a subscriber line, is reflected by the network and an attenuated version is seen as an input signal at the source. That is, an incoming signal received at the near end of the line will include both the signal originating from far end of the line (i.e., the desired message) and a portion of the signal previously sent at the near end of the line (i.e., an echo). This echo is present and deleteriously affects both data and voice transmission quality.
Telephone subscriber line cards provide the interface between the subscriber and the central office of the telephone company. Commonly, these line cards include circuitry adapted to remove or reduce the echo component to enhance signal quality. A typical subscriber line is a two-wire twisted pair cable. Within the central office, a four-wire arrangement is used to form a balancing network. The fundamental problem addressed by the echo control circuit of a telephone subscriber line card is the mismatch in the impedance characteristics of the two-wire subscriber loop and the balancing network. Because of the mismatch, some energy from the received far-end signal is reflected back at the 2 to 4 wire hybrid and turned into an unwanted near-end echo. To maximize the attenuation of a near-end echo various techniques have been employed.
A typical line card includes a subscriber line interface circuit (SLIC) coupled to the line and a coder/decoder (codec) circuit coupled to the SLIC. The codec receives the analog output of the SLIC and samples the output to generate a digital representation of the signal. The digital information is generally passed to a digital signal processor (DSP) for further processing (e.g., extraction of digital data). Digital data received from the DSP is converted to an analog form by the codec and subsequently sent back to the SLIC for transmission on the subscriber line.
One technique for canceling near-end echoes involves using a voltage summing circuit between the codec and the SLIC to scale the voltage of the signal being transmitted and subtracting that scaled voltage from the voltage of the input signal to cancel the echo. If the scaling factor applied to the transmit signal corresponds to the attenuation seen in the transmission line (i.e., the transmit signal travels from the near end to the far end and a portion is reflected back), the echo is reduced.
The voltage summing technique has several limitations. First, there is a delay imparted on the echo portion as it travels between the near and far ends. In voice band frequencies and lower frequency data bands this delay can be tolerated. However, as the frequency of the data signal increases (e.g., >100 kHz), this delay is significant and degrades the effectiveness of the echo canceling technique. A second limitation is that during the passage of the transmit signal through the SLIC, through the subscriber line, and back through the SLIC, noise and non-linearities are introduced. The original transmit signal does not contain noise and/or non-linearities. Hence, subtracting the original transmit signal from the receive signal including the echo leaves behind the noise and non-linearities.
Another technique employed to cancel echoes involves using digital signal processing algorithms to implement an adaptive digital filter. This technique requires that resources of the DSP be used to perform the adaptive filtering. In applications where the resource demands on the DSP are already great, such as in complex communications algorithms (e.g., asynchronous digital subscriber line), there are not sufficient resources remaining to implement a digital echo cancellation technique without compromising the data handling functions.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.